Geochemical and Dietary Drivers of Mercury Bioaccumulation in Estuarine Benthic Invertebrates

Sediments represent
the main reservoir of mercury (Hg) in aquatic
environments and may act as a source of Hg to aquatic food webs. Yet,
accumulation routes of Hg from the sediment to benthic organisms are
poorly constrained. We studied the bioaccumulation of inorganic and
methylmercury (HgII and MeHg, respectively) from different
geochemical pools of Hg into four groups of benthic invertebrates
(amphipods, polychaetes, chironomids, and bivalves). The study was
conducted using mesocosm experiments entailing the use of multiple
isotopically enriched Hg tracers and simulation of estuarine systems
with brackish water and sediment. We applied different loading regimes
of nutrients and terrestrial organic matter and showed that the vertical
localization and the chemical speciation of HgII and MeHg
in the sediment, in combination with the diet composition of the invertebrates,
consistently controlled the bioaccumulation of HgII and
MeHg into the benthic organisms. Our results suggest a direct link
between the concentration of MeHg in the pelagic planktonic food web
and the concentration of MeHg in benthic amphipods and, to some extent,
in bivalves. In contrast, the quantity of MeHg in benthic chironomids
and polychaetes seems to be driven by MeHg accumulation via the benthic
food web. Accounting for these geochemical and dietary drivers of
Hg bioaccumulation in benthic invertebrates will be important to understand
and predict Hg transfer between the benthic and the pelagic food web,
under current and future environmental scenarios.